It has long been recognized that parasitic microorganisms possess the ability to infect animals thereby causing disease and often death. Pathogenic agents have been a leading cause of death throughout history and continue to inflict immense suffering. Though the last hundred years have seen dramatic advances in the prevention and treatment of many infectious diseases, complicated host-parasite interactions still limit the universal effectiveness of therapeutic measures. Difficulties in countering the sophisticated invasive mechanisms displayed by many pathogenic organisms are evidenced by the resurgence of various diseases such as tuberculosis, as well as the appearance of numerous drug resistant strains of bacteria and viruses.
Among those pathogenic agents of major epidemiological concern, intracellular bacteria have proven to be particularly intractable in the face of therapeutic or prophylactic measures. Intracellular bacteria, including the genus Mycobacterium, complete all or part of their lifecycle within the cells of the infected host organism rather than extracellularly. Around the world, intracellular bacteria are responsible for untold suffering and millions of deaths each year. Tuberculosis is the leading cause of death from a single disease agent worldwide, with 8 million new cases and 2 million deaths annually. In addition, intracellular bacteria are responsible for millions of cases of leprosy. Other debilitating diseases transmitted by intracellular agents include cutaneous and visceral leishmaniasis, American trypanosomiasis (Chagas disease), listeriosis, toxoplasmosis, histoplasmosis, trachoma, psittacosis, Q-fever, legionellosis, anthrax and tularemia.
Currently it is believed that approximately one-third of the world's population is infected by Mycobacterium tuberculosis resulting in millions of cases of pulmonary tuberculosis annually. More specifically, human pulmonary tuberculosis primarily caused by M. tuberculosis is a major cause of death in developing countries. Mycobacterium tuberculosis is capable of surviving inside macrophages and monocytes, and therefore may produce a chronic intracellular infection. Mycobacterium tuberculosis is relatively successful in evading the normal defenses of the host organism by concealing itself within the cells primarily responsible for the detection of foreign elements and subsequent activation of the immune system. Moreover, many of the front-line chemotherapeutic agents used to treat tuberculosis have relatively low activity against intracellular organisms as compared to extracellular forms. These same pathogenic characteristics have heretofore limited the effectiveness of immunotherapeutic agents or immunogenic compositions against tubercular infections.
Recently, tuberculosis resistance to one or more drugs was reported in 36 of the 50 United States. In New York City, one-third of all cases tested was resistant to one or more major drugs. Though non-resistant tuberculosis can be cured with a long course of antibiotics, the outlook regarding drug resistant strains is bleak. Patients infected with strains resistant to two or more major antibiotics have a fatality rate of around 50%. Accordingly, safe and effective immunogenic compositions against multi-drug resistant strains of M. tuberculosis are sorely needed.
Initial infections of M. tuberculosis almost always occur through the inhalation of aerosolized particles as the pathogen can remain viable for weeks or months in moist or dry sputum. Although the primary site of the infection is in the lungs, the organism can also cause infection of nearly any organ including, but not limited to, the bones, spleen, kidney, meninges and skin. Depending on the virulence of the particular strain and the resistance of the host, the infection and corresponding damage to the tissue may be minor or extensive. In the case of humans, the initial infection is controlled in the majority of individuals exposed to virulent strains of the bacteria. The development of acquired immunity following the initial challenge reduces bacterial proliferation thereby allowing lesions to heal and leaving the subject largely asymptomatic.
When M. tuberculosis is not controlled by the infected subject it often results in the extensive degradation of lung tissue. In susceptible individuals, lesions are usually formed in the lung as the tubercle bacilli reproduce within alveolar or pulmonary macrophages. As the organisms multiply, they may spread through the lymphatic system to distal lymph nodes and through the blood stream to the lung apices, bone marrow, kidney and meninges surrounding the brain. Primarily as the result of cell-mediated hypersensitivity responses, characteristic granulomatous lesions or tubercles are produced in proportion to the severity of the infection. These lesions consist of epithelioid cells bordered by monocytes, lymphocytes and fibroblasts. In most instances a lesion or tubercle eventually becomes necrotic and undergoes caseation (conversion of affected tissues into a soft cheesy substance).
While M. tuberculosis is a significant pathogen, other species of the genus Mycobacterium also cause disease in animals including man and are clearly within the scope of the present invention. For example, M. bovis is closely related to M. tuberculosis and is responsible for tubercular infections in domestic animals such as cattle, pigs, sheep, horses, dogs and cats. Further, M. bovis may infect humans via the intestinal tract, typically from the ingestion of raw milk. The localized intestinal infection eventually spreads to the respiratory tract and is followed shortly by the classic symptoms of tuberculosis. Another important pathogenic species of the genus Mycobacterium is M. leprae that causes millions of cases of the ancient disease leprosy. Other species of this genus which cause disease in animals and man include M. kansasii, M. avium intracellulare, M. fortuitum, M. marinum, M. chelonei, and M. scrofulaceum. The pathogenic mycobacterial species frequently exhibit a high degree of homology in their respective DNA and corresponding protein sequences and some species, such as M. tuberculosis and M. bovis, are highly related.
Attempts to eradicate tuberculosis using immunogenic compositions was initiated in 1921 after Calmette and Guérin successfully attenuated a virulent strain of M. bovis at the Institut Pasteur in Lille, France. This attenuated M. bovis became known as the Bacille Calmette Guérin, or BCG for short. Ninety years later, immunogenic compositions derived from BCG remain the only prophylactic therapy for tuberculosis currently in use. In fact, all BCG immunogenic compositions available today are derived from the original strain of M. bovis developed by Calmette and Guérin at the Institut Pasteur.
The World Health Organization considers the BCG immunogenic compositions an essential factor in reducing tuberculosis worldwide, especially in developing nations. In theory, the BCG immunogenic composition confers cell-mediated immunity against an attenuated mycobacterium that is immunologically related to M. tuberculosis. The resulting immune response should inhibit primary tuberculosis. Thus, if primary tuberculosis is inhibited, latent infections cannot occur and disease reactivation is avoided.
Current BCG immunogenic compositions are provided as lyphophilized cultures that are re-hydrated with sterile diluent immediately before administration. The BCG immunogenic composition is given at birth, in infancy, or in early childhood in countries that practice BCG vaccination, including developing and developed countries. Adult visitors to endemic regions who may have been exposed to high doses of infectious Mycobacteria may receive BCG as a prophylactic providing they are skin test non-reactive. Adverse reactions to the immunogenic composition are rare and are generally limited to skin ulcerations and lymphadenitis near the injection site. However, in spite of these rare adverse reactions, the BCG immunogenic composition has an unparalleled history of safety with over four billion doses having been administered worldwide since 1930.
However, the unparalleled safety of traditional BCG immunogenic compositions is coming under increased scrutiny and has created a paradox for healthcare practitioners. The population segments most susceptible to mycobacterial infections are the immunocompromised and immunosuppressed. Persons suffering from early or late-stage HIV infections are particularly susceptible to infection. Unfortunately, many persons in the early-stage of HIV infection are unaware of their immune status. It is likely that these individuals may voluntarily undergo immunization using a live attenuated immunogenic composition such as BCG without being forewarned of their unique risks. Moreover, other mildly immunocompromised or immunosuppressed individuals may also unwittingly undergo immunization with BCG hoping to avoid mycobacterial disease. Therefore, safer, more efficacious BCG and BCG-like immunogenic compositions are desirable.
Recently, significant attention has been focused on using transformed BCG strains to produce immunogenic compositions that express various cell-associated antigens. For example, C. K. Stover, et al. have reported a Lyme Disease immunogenic composition using a recombinant BCG (rBCG) that expresses the membrane associated lipoprotein OspA of Borrelia burgdorferi. Similarly, the same author has also produced a rBCG immunogenic composition expressing a pneumococcal surface protein (PsPA) of Streptococcus pneumoniae. (Stover C K, Bansal G P, Langerman S, and Hanson M S. 1994. Protective immunity elicited by rBCG immunogenic compositions. In: Brown F. (ed): Recombinant Vectors in Immunogenic composition Development. Dev Biol Stand. Dasel, Karger, Vol. 82:163-170)
U.S. Pat. No. 5,504,005 (the “'005” patent”) and U.S. Pat. No. 5,854,055 (the “'055 patent”) both issued to B. R. Bloom et al., disclose theoretical rBCG vectors expressing a wide range of cell associated fusion proteins from numerous species of microorganisms. The theoretical vectors described in these patents are either directed to cell-associated fusion proteins, as opposed to extracellular non-fusion protein antigens, and/or the rBCG is hypothetically expressing fusion proteins from distantly related species.
Furthermore, neither the '005 nor the '055 patent disclose animal model safety testing, immune response development or protective immunity in an animal system that closely emulates human disease. In addition, only theoretical rBCG vectors expressing M. tuberculosis fusion proteins are disclosed in the '005 and '055 patents; no actual immunogenic compositions are enabled. Those immunogenic composition models for M. tuberculosis that are disclosed are directed to cell-associated heat shock fusion proteins, not extracellular non-fusion proteins.
U.S. Pat. No. 5,830,475 (the “'475 patent”) also discloses theoretical mycobacterial immunogenic compositions used to express fusion proteins. The immunogenic compositions disclosed are intended to elicit immune responses in non-human animals for the purpose of producing antibodies thereto and not shown to prevent intracellular pathogen diseases in mammals. Moreover, the '475 patent does not disclose recombinant immunogenic compositions that use protein specific promoters to express extracellular non-fusion proteins.
U.S. Pat. No. 6,467,967 claims immunogenic compositions comprising a recombinant BCG having an extrachromosomal nucleic acid sequence comprising a gene encoding a M. tuberculosis 30 kDa major extracellular protein (also known as Antigen 85B), wherein the M. tuberculosis 30 kDa major extracellular protein is over-expressed and secreted. Moreover, U.S. Pat. No. 6,924,118 claims additional recombinant BCG that over-express other M. tuberculosis major extracellular proteins.
Therefore, there remains a need for recombinant intracellular pathogen immunogenic compositions that induce protective immune responses.